1. Field of the Invention
This invention relates to a novel polyaryl ether sulfone semipermeable membrane and a process for producing the same.
2. Description of the Prior Art
Many high molecular weight compounds are used for the production of semipermeable membranes, which include for example cellulose acetate, polyacrylonitrile and polyamide. Polyaryl ether sulfone is also used for the semipermeable membrane application because of its good resistance to heat and chemicals, although it has primarily been used as engineering plastics. Polyaryl ether sulfone semipermeable membranes are disclosed in U.S. Pat. Nos. 3,567,810, 3,615,024, 3,639,306 and 3,691,068, assigned to Amicon Corporation; U.S. Pat. No. 4,051,300 assigned to Gulf South Research Institute; U.S. Pat. No. 3,709,841 assigned to Phone-Poulenc S.A.; U.S. Pat. Nos. 3,762,136 and 3,709,774, and British Pat. No. 1,295,585 assigned to General Electric Co., Tomoegawa Paper-Manufacturing Co.'s Japanese Laid-open Patent Application 28,581/1976, and; Journal of Applied Polymer Science, 20, 2377-2394 and 2395-2406(1976), ibid 21, 1883-1900(1977). Of these polyaryl ether sulfone semipermeable membranes, Gulf Research Institute's membranes were developed for use in reverse osmosis and are characterized as exhibiting a very low permeability to water, although pores on both surfaces of the membranes are large, i.e., 250 angstroms to 0.44 micron in pore diameter. The water permeability of the membranes is at most 1.3 m.sup.3 /m.sup.2.day.kg/cm.sup.2 and, hence, of little practical use in ultrafiltration. The known semipermeable membranes other than Gulf Research Institute's membranes are characterized as possessing a thin relatively dense surface layer on the adjacent first major surface of the membranes, but no such surface layer on the second major surface thereof. Since voids of a diameter exceeding 10 microns are exposed on the second major surface, these known semipermeable membranes have the defects that:
(1) they are poor in mechanical strength;
(2) they are difficult or even impossible to back-wash; and
(3) they are liable to be plugged or fouled.
In general, semipermeable membranes made of a relatively hydrophilic polymer, such as polyacrylonitrile or sulfonated polysulfone, exhibit a large permeability to water. In contrast, semipermeable membranes made of a relatively hydrophobic polymer, such as polyaryl ether sulfone, exhibit a small permeability to water. If it is intended to provide polyaryl ether sulfone semipermeable membranes of an increased water permeability, the membranes must be of a special structure, such as, for example, an anisotropic structure comprised of a dense surface layer and a support layer integrated with the surface layer. Such an anisotropic structure is often seen in polyacrylonitrile, sulfonated polysulfone and cellulose acetate semipermeable membranes. The surface layer of the anisotropic semipermeable membrane is a thin, relatively dense barrier layer defining the permeability to water. The support layer of the anisotropic semipermeable membrane is a much more coarsely porous layer through which fluid can pass with little hydraulic resistance. However, such an anisotropic semipermeable membrane is not able to withstand the hydraulic compressional force very well, is liable to become consolidated and, also is poor in mechanical strength.